U.S. Pat. No. 4,923,657 discloses a powder casting process in which pigmented plastic particles of thermoplastic material are deposited on a heated surface to form a thin plastic shell or skin. While suitable for its intended purpose, the process disclosed in the '657 patent uses thermoplastic particles having dimensions in the range of 0.002" to 0.016". The particles disclosed in the '657 patent disclosure were virgin polyvinyl chloride resin; plasticizers; stabilizers and pigments blended together in a high intensity mixer. A typical high intensity mixer is nothing more than a heavy duty household type mixing blender. The blender operates at a speed that will cause frictional heating of the resin. In such processes the virgin PVC resin started with a particle having a diameter between 120 to 140 microns. The PVC particle is malleable and has microscopic cracks or fissures that are observable on the surface of the particles. In fact, because the particles are made during the polymerization process, they are found to be formed of ever smaller round particles that are full of passages into which the plasticizers and additives can wick during high intensity blending. Initially, during blending in the high intensity mixer, approximately 50% of the blending plasticizer is added to the mixer. After high intensity mixing, the PVC resin particle softens and swells to a diameter of 180 to 355 microns. In a typical high intensity mixing process the temperature of the material increases from ambient to approximately 180.degree. F. At this temperature the PVC particles can absorb higher concentrations of plasticizers and additives. Since pigments and stabilizers are the most difficult constituents to fully disperse into the resin particle, they are usually the last to be added during a typical compounding cycle. Once they are added the compounding temperature is raised to the peak temperature below which the PVC resin particle will not melt so as to form particle agglomerates. The maximum mix temperature is dependent upon the molecular weight of the PVC and the type of plasticizer that is used. But in all cases, the characteristic of particles formed by high intensity mixer compounding the resin particle continues to soften and grow in size as the plasticizers and additives are absorbed or diffused into the resin particle. After the peak temperature is reached (above which melting occurs) the blended material is transferred to a cooling vessel that is equipped with a large blade that rotates at a low speed, for example, 20 RPM, that stirs the compounded material over the cooling surfaces of the vessel without generating any additional frictional heat. As the compounded material cools, the resin particles begin to contract so as to hold the plasticizers and additives within the resin particles. Such particles generally are heat sensitive in use and under high temperature ambient conditions, e.g., plant temperatures greater than 80.degree. F., the material softens and can become sticky making it difficult to use in roto-casting and other powder casting processes. The softened material becomes tacky and this in turn can lead to manufacturing problems including bridging between the particles, the presence of holes in skins cast from the material and difficulty in controlling the total weight of shells manufactured from such material. In some cases it has been necessary to refrigerate the material when ambient temperatures are elevated to avoid such problems.
Other problems arise since the individual particles formed by the high intensity mixer process have a roughened surface with fissures and cracks therein. Such materials often do not flow into tight corners found in many powder casting mold configurations. Other thermoplastic materials used in powder casting or slush molding of thin shell thermoplastic parts include material that is cryogenically ground. Such material can have a wide range of shapes and is also characterized by outer surface configurations that include cracks and fissures that are apparent to the eye or under magnifications less than 10.times..
Use of such irregularly shaped particles in processing that includes feeding the particles to a point of use and casting the particles on a heated mold to melt flow the particles and cool them to form thin plastic shells presents several other problems. In the case of slush molding, the particles are retained in a powder box that is rotated to direct an excess charge of material into a mold cavity. Such thermoplastic, irregular particles do not flow smoothly from all corners of a powder box. Furthermore, such particles do not smoothly flow into all parts of complex shaped molds of the type having tight return passages and very small mold surface features that simulate features such as leather grains, stitching or the like. As a result, it is necessary to vibrate the powder box and molds during the various processing steps so that the particles will flow against heated mold surfaces so as to melt and form a skin or shell shape corresponding to the shape of the heated mold surface. The irregular shape also produces an uneven build-up of material on the heated mold surface such that the particles do not melt and flow uniformly against the heated mold surface. As a consequence, the resultant cast part can have irregular backside build-up when the part is cooled and extracted from the mold surface. Such irregularities on the backside require that the nominal thickness of the part be larger than required for a given application which in turn takes more material than in the case of a part that has a uniform shape on its backside.
Further, it has been found that irregularly shaped particles define an extended surface area that tends to collect moisture so that the particles do not flow smoothly onto a casting surface from conventional powder box apparatus. This problem is especially pronounced in the case of hydrophilic thermostatic material such as thermoplastic urethane material (TPU). Additionally, moisture build-up on such irregularly shaped particles can cause variations in the thermal load on the casting surface so that the temperature of the casting surfaces require continual adjustment making it difficult to control the quality of the finished product. Additionally, moisture content in excess of 0.01% will create porosity during the casting sequence because the excess water will flash-vaporize when it comes in contact with the heated tool surface.
In the past, TPU has also been used in roto-casting thin plastic parts. In such case the TPU material can be manufactured in a clear flake form that is pigmented and then cryogenically ground into a powder form. The powder is very fine and has an irregular, very coarse outer surface with cracks and fissures that define a high surface area that will hold moisture. Such moisture content will vary in the irregularly shaped materials causing the TPU particles to collect in hoppers and other handling material prior to use in a melt molding process in which the thermoplastic material is placed on the heated surface of a shaping mold. In the case of roto-casting the material is a fixed weight charge corresponding more or less to the weight of the finished product. In such processes the charge is selected to correspond to the weight of the finished part and the charge is continuously centrifuged against the mold surface to be melted and flowed to form the thin walled part. Examples of such roto-casting processes and methods are set forth in U.S. Pat. Nos. 4,167,382 and 4,767,299. In the past, irregularly shaped particles used in roto-casting were cryogenically ground from thermoplastic materials such as TPU or were formed by polymerizing polyvinyl chloride.
Pigment microspheres are also known. They can be added to resin particles in a hopper and mixed in the extruder of an injection molding machine to impart color to the resin particles. Such pigment microspheres are comprised primarily of pigment material in a binder of clear polyethylene. In such cases the pigment comprises 50% of the total weight of each of the pigment microspheres. Such pigment particles are added to polymeric material such as PVC and alloys of PVC and mixed therewith in mixers prior to injection into injection molding equipment. Typically, such pigment microspheres have been sold in a size range of 0.040 to 0.080 inches. They are suitable for use with virtually all thermoplastics used for injection molding including polyvinyl chloride, polyethylene, polypropylene, nylon, ABS, polyesters, elastomers and the like and, while suitable for their intended purpose of adding coloring to injection molded parts, have not been suggested for use in various roto-casting or slush molding processes and have not been suitable for such purpose since the microsphere has been comprised of mostly pigment material in a small amount of binder that will not provide desired melting and flow characteristics if used in powder casting processes such as roto-casting or slush molding.
Additionally, it is known to form thermoplastic pellets having a spheroidal shape that are added to other materials in an extruder upstream of injection molding apparatus. Since such particles are mixed and melted with other materials (such as pigment particles) in the extruder they come in a size range of 0.030" and greater. While suitable for use in charging an extruder for use in an injection molding process, such sized particles have not been found suitable for use in powder casting processes such as roto-casting or slush molding since they are too large for molds having tight returns and small dimensioned mold features.